NASA goals for space power arrays call for specific powers for the array in excess of 300 watts/kg so that the mass and drag of the spacecraft power system will be reduced and so that the spacecraft, then, can carry larger payloads. Because the hardware of the array reduces the specific power by adding weight without increasing the power (or solar energy conversion), the cells of the array must have specific powers well in excess of 300 watts/kg if the resulting array will achieve the goal.
Mickelsen and Chen describe thin film, polycrystalline, I-II-VI.sub.2 (i.e. CuInSe.sub.2) semiconductors suitable for space solar cells in United States Patent Reissue No. 31,968 and U.S. Pat. No. 4,523,051 (both incorporated by reference). Their CuInSe.sub.2 /(Cd,Zn)S cells are deposited on relatively thick substrates selected from polycrystalline alumina, glazed alumina, enameled steel, metal foils, and similar inert inorganic materials. Typically, the substrate has a thickness of at least about 25 mils and is preferably either 25 mil alumina or 60 mil soda lime glass. Cells of this type can have efficiencies on the order of 10% AMO, but the specific power of the cells is dramatically reduced by the mass of the substrate.
Presently, silicon solar cells are used for space power applications. I-III-VI.sub.2 semiconductor cells, particularly (Cd,Zn)S/CuInSe.sub.2, would provide several advantages over silicon:
(1) The CuInSe.sub.2 cell is generally only 10 microns thick (without substrate) and, therefore, offers the potential of an extremely high specific power. Conventional silicon cells are 50-100 times thicker.
(2) Radiation testing has shown the CuInSe.sub.2 cell to be at least about 50 times more resistant to 1 MeV protons than silicon cells. The CuInSe.sub.2 cell also possesses an inherent tolerance to irradiation by 1 MeV electrons up to at least 2.times.10.sup.16 electrons/cm.sup.2. At this fluence, typical silicon cells are degraded by over 50%. Because of the radiation hardness of the CuInSe.sub.2 cell, reduced radiation shielding is required with CuInSe.sub.2 cells which results in an even higher specific power. By using the equivalent radiation shielding on CuInSe.sub.2 cells as on silicon cells, a higher end-of-life efficiency can be achieved for CuInSe.sub.2 as well as a higher specific power.
(3) Annealing of the cell, after proton irradiation, at 200.degree. C. for six minutes restores the CuInSe.sub.2 cell to within 95% of its initial efficiency.
The major limiting factor against using CuInSe.sub.2 cells for space applications has been a low specific power for the cells primarily caused by the substrate mass. While soda lime glass or alumina substrates are satisfactory for terrestrial applications, the cells deposited on the substrates possess a low specific power. Therefore, a much lighter substrate is required to achieve NASA's goal and to meet the demands for modern space power applications.
In U.S. Pat. No. 4,703,131, Dursch describes an improved space solar cell having a specific power in excess of 400 watts/kg and comprising a (Cd,Zn)S/CuInSe.sub.2 thin film on a 2-5 mil titanium metal foil. The higher specific power results from a more efficient transducer and a much lighter substrate. Further improvements can be achieved, however, because these solar cells have conversion efficiencies of only about 8-10% AMO.
In U.S. Pat. No. 4,680,422, I describe a two-terminal, thin film, tandem solar cell comprising an upper cell of a heterojunction made from Groups II and VI elements (generally, CdSe) mechanically stacked or physically attached to a CuInSe.sub.2 lower cell. Physical attachment of the CdSe upper cell is achieved through a lattice mismatch transition layer comprising a ZnSe window layer atop the lower cell and a ZnTe layer atop the ZnSe. In the mechanically stacked tandem cell, I suggest that the upper cell could be GaAs or GaAlAs, and that such thin films could be made with the CLEFT process.
Those solar cells would not achieve the maximum obtainable efficiencies or specific powers, which are so important for space applications. I now describe as part of the present invention a monolithic tandem solar cell having a potential specific power of at least about 1000 watts/kg and a conversion efficiency of about 25% AMO.